This invention relates generally to medical imaging systems and, more particularly, to automatically selecting a scan protocol for medical imaging systems.
At least some known medical imaging systems include a set-up procedure to prepare the system for a scan of a patient. For example, a trained medical technologist may receive an order from a doctor for a particular type of scan of a region of interest of the patient. To comply with the doctor's order and properly perform the scan, various instructions may be input into a controller of the medical imaging system used to perform the scan. To facilitate such input, several predetermined protocols may be saved in a memory of the medical imaging system and one or more predetermined protocols may be selected such that the instructions and parameters contained in the protocol may be used to control the medical imaging system during the scan. In the specific example of a PET/CT scan, such protocols generally include, for example, emission frame duration, transmission frame duration, number of frames to acquire, frame overlap and choice of image reconstruction method. Predetermined protocols permit the technologist to make a judgment based on the patient size/weight, history, and indication and then select the predetermined protocol that may match the information desired. For example, a lung cancer indication with the purpose of scanning a whole-body looking for malignant metastases may be selected to image from upper-thigh to top of head, whereas, a different indication may be prescribed to image a different portion of the patient or change an emission duration for the frames. However, such protocols are subject to a judgement of the technologist in their selection and the selection may be between two protocols having less than optimal imaging settings for the patient to be scanned.
Additionally, patient-related imaging conditions, such as strict control of imaging time post-injection, injected activity, physiologic uptake, and patient disease state may affect image quality by affecting the dynamic range of imaging of true, random and scatter coincidences.
At least some known imaging systems, for example, a PET imaging system, a rate of prompt (true, scatter, random) coincidence events and random coincidence events are measured per second in real-time. Such rates may be a function of the imaged object size, composition, radiation source distribution, imaging system collimation, detector or scanner axial acceptance angle and radioactivity concentration. As used herein, axial acceptance refers to the maximum out-of-plane angle at which data will be accepted; in a PET detector consisting of stacked rings of detector crystals, axial acceptance corresponds to the maximum accepted ring difference for valid data. Such known imaging systems may set the acceptance angle based upon the dimensionality of the acquisition mode, for example, two dimensional or three dimensional. These fixed values may be based on imaging phantoms or other objects and then analyzing the resultant image quality to determine a reasonable acceptance angle setting. Alternatively, the acceptance angle may be based on results from simulation studies for simple objects, such as uniform cylinders. Such an acceptance angle determination method is not based on the characteristics of either the object currently being imaged or the prompt versus random coincidence rates currently being detected in the imaging system. Various scan parameters, including the axial acceptance angle, coincidence timing window, energy window, and the use (and if used, the configuration) of slice septa are selected by an operator prior to initiation of a patient imaging scan. However, such settings are not based on current conditions during the imaging scan, and as such, the imaging system may not be operating in an optimal manner, as measured by metrics, such as noise-equivalent-count-rate (NECR).